It is known in the art of microwave generation that transit-time, transferred electron and field effect transistor devices are efficient sources of microwave energy. Large signal behavior of transit-time devices was postulated in the late 1960's and Si and GaAs devices based on this theory were demonstrated in the late 1970's and early 1980's and have been more fully described in such treatises as "Physics of Semiconductor Devices," Sze et al, Wiley Interscience, 1981. The fundamental features common to these devices is the direct proportionality between the operating frequency and saturation velocity, and the inverse proportionality between frequency and depletion region width.
Specific examples of these transit time devices are IMPATT (impact ionization avalanche and transit time device) diodes and a BARITT (barrier inject transit time device) diodes. IMPATT and BARITT diodes are typically used as high frequency devices which amplify high frequency waves into high power waves or generate an oscillation of high frequency waves with high power. IMPATT diodes typically include an n+ type (or p+ type) semiconductor layer, a less doped semiconductor layer, an n-type (or p-type) semiconductor layer and a p+ type (or n+ type) semiconductor layer, all of which make a strata in this order. Electrodes are fitted on both ends (n+ type and p+ type of layers) of the strata. In operation, a reverse bias voltage is applied to the diode, that is, the electrode of the n+ type layer is connected to the positive terminal of an electric power source, and the electrode of the p+ type layer is connected to the negative terminal of the power source. This reverse bias voltage induces a carrier avalanche in the less doped semiconductor layer near the n-type layer. The electrons generated by the avalanche run through the less doped semiconductor layer to the n+ type layer with saturated velocity. This phenomenon induces negative resistance in the diode. The occurrence of negative resistance enables the diode to generate microwave oscillation. A typical IMPATT diode has a pn-junction. But there are other types of IMPATT diodes in which the pn-junction is replaced by a Schottky junction between a metal and a semiconductor.
BARITT diodes typically have a structure in which a metal layer, a p-type layer (or n-type layer) of semiconductors and a metal layer make strata in this order. One type of junction between the metal and one semiconductor layer is a Schottky junction. Similar to IMPATT diodes, when a reverse bias voltage is applied to the diode, majority carriers are injected to the semiconductor layer. The action of the majority carriers generates microwave oscillation. Also like IMPATT diodes BARITT diodes can have a pn-junction instead of the Schottky junction between the metal and the semiconductor.
The semiconductor material of these high frequency devices (IMPATT and BARIT diodes) has in the past been typically silicon or gallium arsenide. Other semiconductor materials have not been used as the material of choice for these high frequency devices so far, although diamond has been shown to lower the heat in such transit time devices based on silicon and gallium arsenide. Such a device made from diamond is disclosed in U.S. Pat. No. 5,243,119, issued to Shiomi et al on Sep. 7, 1993.
Unfortunately, known transit-time oscillator devices based on silicon and gallium arsenide are, at best, capable of generating only gigahertz range frequency signals. Therefore, it would be desirable to produce transit-time oscillator devices capable of generating terahertz range frequency signals.